Microwave noise generator



Sept. 8, 1964 P. A. H. HART 3,148,341

MIRcEowAvE NoIsE GENERATOR Filed Dec. 5, 1961 2O TI INVENTOR PAUL A. H. HART BY ,e

AGEN

United States Patent O 3,148,341 MICROWAVE NOlSE GENERATOR Paul Anton Herman Hart, Emmasingel, Eindhoven,

Netherlands, assignor to North American Philips Company, Inc., New York, NSY., a corporation of Delaware Filed Dec. 5, 1961, Ser. No. 157,167 Claims priority, application Netherlands Jan. 19, 1961 6 Claims. (Cl. 331-78) This invention relates to a noise generator for microwaves, in which the microwave radiation produced in the positive column of a gas discharge is collected in a cavity resonator.

In a known noise generator for the centimeter Waverange a uorescent discharge lamp is arranged at right angles to the longitudinal direction of a hollow waveguide, parallel to the largest dimension of the cross section, across the cavity resonator. The discharge is performed substantially completely in mercury vapour, which involves a strong dependence on temperature. With a different structure the gas discharge tube is taken obliquely through the hollow waveguide. In both cases leakage to the outside must be prevented by means of long sleeves at lthe waveguide openings around the discharge tube.

The plasma of the positive column may be considered in the microwave range as a source of black radiation with a very high equivalent temperature. This radiation is produced during the collision of electrons. The collision frequency should not be excessively low with respect to the frequency at which the noise is measured. If, indeed, the noise associated with the equivalent temperature should be measured, the plasma has to give olf its noise energy completely to the waveguide, which requires a complete impedance match between the noise source and the waveguide.

For millimeter waves it is practically impossible to manufacture a gas discharge tube by the existing technological means to serve as a source of noise in the known arrangement. With the restricted choice of discharge conditions (tube diameter and dissipation) it is furthermore ditiicult to attain a suiciently high collision frequency of the electrons. A satisfactory matching is therefore not possible.

In a noise generator for microwaves, in which the microwave radiation produced in the positive column of a discharge in a gas is collected in a wave-guide section, the gas discharge tube comprises, in accordance with the invention, an elongated mica window in a rectangular -aperture in a metal tube, which window is engaged by a horn antenna having its longitudinal dimension parallel to the tube axis, this antenna going over into the hollow waveguide having a rectangular section for the noise frequenmy range having wave-lengths below a predetermined wave-length.

With the structure according to the invention an irnpedance match between the noise source and the waveguide is obtained in a simple manner without :the dimensions of the tube preventing the formation of a positive column. The tube may have a fairly large diameter, which Vinvolves structural advantages.

With an excessively small depth of Ithe discharge, viewed from the window, the measured noise temperature would remain markedly below the attainable temperature. With a particular embodiment of the invention the arrangement is therefore such that opposite the window a reector having low losses is arranged. This reector may be formed by a satisfactorily conductive wall portion or by a separately arranged copper plate. In order to avoid disturbance of the discharge field, the

3,148,341 Patented Sept. 8, 1964 ICC satisfactorily conductive wall or the separately arranged plate must be enameled.

In certain cases it may be a disadvantage that with the gas discharge extinguished the horn antenna is no longer closed free of reflection. In accordance with ythe invention this may be solved by arranging a second mica window opposite the first mica window, a horn antenna being also arranged on this second window, which antenna terminates in a reflection-free wedge.

The invention will be described more fully with reference to Ithe accompanying drawing, in which FIGS, l and 2 show a sectional view of a noise generator according to the invention along the axis and at right angles to the axis of the gas discharge tube respectively and FIG. 3 is a cross sectional view of one embodiment in which impedance matching is satisfactory even in the extinguished state of the gas discharge.

Referring to FIGS. 1 and 2, reference numeral 1 designates a ferrochromium central portion of a gas discharge tube, coated on the inner side with an enamel layer 2. The diameter is about 25 mm. In the glass portions 3 and 4 are arranged [the incandescent heating cathode 5 and the flat nickel anode 6 respectively. In a flat side of the portion 1 provision is made of a mica window 7, which is secured in a vacuum-tight manner by means oi' enamel. The setting of the mica window is engaged by the horn antenna 8, which goes over in the hollow waveguide 9, provided with a connecting liange 10. The ribs of the horn antenna are stiifened as indicated at 11. FIG. 2 furthermore shows the at, conductive mirror 12 with an enamel layer 13 which, in FIG. l is omitted. As shown in FIG. 3 a second window 17 with a horn antenna 18 and a hollow waveguide 19 having a reflectionfree wedge 2S of lossy material on the closing plate 21 are arranged opposite the mica window 7.

lf the discharge takes place in rare gas, e.g. argon, the equivalent temperature (of a sourse of black radiation) is of the theoretically attainable value up to a wavelength of about 8 mm. With 4 mm. this value is not more than 60% A suitable rare gas pressure is about 10 mm. The tube diameter may be chosen considerably smaller.

The tube may, as an alternative, be filled with a nonrare gas, e.g. nitrogen for the examination of the properties thereof, so that universal use is obtained.

What is claimed is:

l. A noise generator for microwaves having wavelengths below a predetermined wave-length comprising an electric discharge tube including an envelope containing an ionizable gaseous medium at a pressure at which a positive column discharge can be sustained and a pair of electrodes for producing the discharge, said envelope having a longitudinal axis, at least a portion ot' the envelope constituting a wave-guide section having dimensions substantially greater than said predetermined wave-length, said cavity resonator portion of the envi:- lope having a window therein transparent to microwave energy, a portion of Ithe internal surface of the envelope surrounding the window being covered with a material which avoids disturbance of the discharge ield in said wave-guide section, a wave-guide having a rectangular section for the frequency range of said microwaves, and means to couple said wave-guide to said window, said latter means comprising a horn antenna having an outwardiy haring end portion engaging said window and a smaller end connected to said wave-guide, the largest dimension of said outwardly aring end being parallel to the longitudinal axis of the tube.

2. A noise generator as claimed in claim l in which the window is constituted of mica.

9 A LE L 3. A noise generator as claimed in claim 2 in which gaged by a horn antenna which terminates in a reectionsaid material surrounding the window is enamel. free wedge.

4. A noise generator as claimed in claim 3 in which the inner Wall of the tube opposite the window is conductive. References Cited m the me of this patent 5. A noise generator as claimed in claim 4 in which the 5 UNITED STATES PATENTS Wall opposite the Window is covered with enamel. 2,474,384 Sunstein June 28, 1949 6. A noise generator as claimed in claim 1 in which a 2,617,937 Van Atta Nov. 11, 1952 second Window is provided in the Wall of the tube 0p- 2,706,782 Mumford Apr. 19, 1955 posite the irst window, said second Window being en- 2,942,204 Poulter June 21, 1960 

1. A NOISE GENERATOR FOR MICROWAVES HAVING WAVELENGTHS BELOW A PREDETERMINED WAVE-LENGTH COMPRISING AN ELECTRIC DISCHARGE TUBE INCLUDING AN ENVELOPE CONTAINING AN IONIZABLE GASEOUS MEDIUM AT A PRESSURE AT WHICH A POSITIVE COLUMN DISCHARGE CAN BE SUSTAINED AND A PAIR OF ELECTRODES FOR PRODUCING THE DISCHARGE, SAID ENVELOPE HAVING A LONGITUDINAL AXIS, AT LEAST A PORTION OF THE ENVELOPE CONSTITUTING A WAVE-GUIDE SECTION HAVING DIMENSIONS SUBSTANTIALLY GREATER THAN SAID PREDETERMINED WAVE-LENGTH, SAID CAVITY RESONATOR PORTION OF THE ENVELOPE HAVING A WINDOW THEREIN TRANSPARENT TO MICROWAVE ENERGY, A PORTION OF THE INTERNAL SURFACE OF THE ENVELOPE SURROUNDING THE WINDOW BEING COVERED WITH A MATERIAL WHICH AVOIDS DISTURBANCE OF THE DISCHARGE FIELD IN SAID WAVE-GUIDE SECTION, A WAVE-GUIDE HAVING A RECTANGULAR SECTION FOR THE FREQUENCY RANGE OF SAID MICROWAVES, AND MEANS TO COUPLE SAID WAVE-GUIDE TO SAID WINDOW, SAID LATTER MEANS COMPRISING A HORN ANTENNA HAVING AN OUTWARDLY FLARING END PORTION ENGAGING SAID WINDOW AND A SMALLER END CONNECTED TO SAID WAVE-GUIDE, THE LARGEST DIMENSION OF SAID OUTWARDLY FLARING END BEING PARALLEL TO THE LONGITUDINAL AXIS OF THE TUBE. 